Coating processes – Coating remains adhesive or is intended to be made adhesive – Pressure sensitive adhesive
Reexamination Certificate
2000-08-17
2003-01-14
Pianalto, Bernard (Department: 1762)
Coating processes
Coating remains adhesive or is intended to be made adhesive
Pressure sensitive adhesive
C427S294000, C427S428010, C427S496000, C427S551000
Reexamination Certificate
active
06506447
ABSTRACT:
The present invention relates to a process for the continuous, solvent- and mastication-free production of pressure-sensitive self-adhesive compositions based on non-thermoplastic elastomers, using tackifier resins, typical rubber plasticizers, optionally fillers and heat-activatable crosslinkers, and to the coating thereof to produce self-adhesive articles, in particular high-performance self-adhesive tapes.
Fundamental to adhesive systems and the pressure-sensitive adhesive articles produced with them are the two physical phenomena of adhesion and cohesion. Adhesion is dealt with in the technical jargon using the terms instant bond strength (tack) and bond strength (peel strength) and describes by definition the terms “self-adhesive” and/or “pressure-sensitive adhesive”, i.e. permanent adhesive bonding under “gentle pressure”.
Especially in the case of pressure-sensitive adhesives based on natural rubber, this property is obtained by mixing in tackifying resins (tackifiers) and plasticizers having relatively low molecular weights.
The second defining property of the pressure-sensitive adhesives is their simple residue-free redetachability after use. This property is determined essentially by the high molecular mass rubber fractions as the elastomer component, which give the system, in the form of cohesion (internal strength), the required strength under shear stress, which is of particular significance for the use of the products at relatively high temperatures and/or mechanical loads. By additional crosslinking, for example by way of ionizing rays, reactive resin components or other chemical crosslinkers, this property can be reinforced.
The performance of the pressure-sensitive adhesive is, therefore, critically determined by the balanced proportion of adhesion properties and cohesion properties and by compatibility, homogeneity and stability of the blend of components with extremely high and relatively low average molecular weights, something which is relatively easy to achieve in the case of production of the composition in industry-standard mixers and kneading machines using solvents.
The solvent-free compounding and processing of self-adhesive compositions, on the other hand, has become established primarily only for the processing of elastomers which melt, so-called thermoplastic elastomers.
In this case, the process of producing the composition is normally conducted in the melt in twin-screw extruders at relatively high temperatures, with coating taking place normally by means of slot dies.
The advantage of using thermoplastic elastomers lies essentially in a simplification of the coating process. The avoidance of flammable solvents does away with the need for the drier units, with their high energy consumption for the evaporation and recovery of the solvents, and to use explosion-protected units. Hot-melt coating units are compact and permit much higher coating speeds. Moreover, the technology is an environment-friendly one in which there are no solvent emissions.
For the solvent-free compounding of thermoplastic elastomers, the prior art makes use predominantly of block copolymers having polystyrene block fractions. The advantage of this class of substance is that the polystyrene domains present in the polymer soften above 100° C., accompanied by a sharp fall in the viscosity of the adhesive composition and thereby providing ease of processing. After cooling to room temperature, the polystyrene domains are reformed and impart a certain shear strength to the pressure-sensitive adhesives based on thermoplastic elastomers.
The thermoplastic elastomers can be compounded faultlessly in the extruder process using hydrocarbon resins which promote bond strength. In this way, a desired level of bond strength can be achieved with relative ease. The resultant pressure-sensitive adhesives, however, remain sensitive to temperatures above 40° C. For the self-adhesive tapes produced on this basis, this remanent “creep behaviour” is critical for unrestricted storage stability (blocking of the rolls in the stack, especially in the course of transportation in relatively warm climate zones) and for their use at relatively high operating temperatures (for example as masking tapes in automotive finishing, where despite postcrosslinking such tapes lose their functional capacity: the pressure-sensitive adhesive softens and the shear strength for fixing the masking papers is no longer ensured).
For this reason, the known hot-melt pressure-sensitive adhesives based on block copolymers have been able to establish themselves almost exclusively for packaging tapes and for labels for use at room temperatures.
Using non-thermoplastic elastomers, such as natural rubber, on the other hand, it is possible to achieve the required shear strengths; however, the solvent-free production and processing of natural-rubber pressure-sensitive adhesives has to date confronted the person skilled in the art with unsolved problems.
Owing to the extremely high molecular mass fractions of the rubber (with M
w
≧1 million), solvent-free self-adhesive compositions cannot be processed by the hot-melt pressure-sensitive adhesive technology, or else the rubbers used must be reduced in their molecular weight (broken down) so greatly before processing that as a result of this breakdown their suitability for high-performance self-adhesive compositions is impaired.
The deliberate industrial process of rubber breakdown under the combined action of shear stress, temperature and atmospheric oxygen is referred to in the technical literature as mastication and is generally carried out in the presence of chemical auxiliaries, which are known from the technical literature as masticating agents or peptizers, or, more rarely, as “chemical plasticizing aids”.
In rubber technology, the mastication step is necessary in order to make it easier to integrate the additives.
According to Rompp (Rompp Lexikon Chemie—Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998), mastication is a term used in rubber technology for the breaking down of long-chain rubber molecules to increase the plasticity and/or reduce the (Mooney) viscosity of rubbers. Mastication is carried out by treating, in particular, natural rubber in kneading apparatus or between rolls at very low temperatures in the presence of mastication aids (masticating auxiliaries). The high mechanical forces which act lead to a “tearing apart” of the rubber molecules, with the formation of macroradicals, whose recombination is prevented by reaction with atmospheric oxygen. Mastication aids such as aromatic or heterocyclic mercaptans and their zinc salts or disulphides accelerate the mastication process by promoting the formation of primary radicals. Activators such as metal (iron, copper, cobalt) salts of tetraazaporphyrins or phthalocyanines enable the mastication temperature to be lowered. For the mastication of natural rubber, mastication aids are used in amounts of from about 0.1 to 0.5% by weight in the form of masterbatches, which facilitate a uniform distribution of this small amount of chemicals within the rubber composition.
Mastication must be clearly distinguished from the breakdown known as degradation which results in all of the standard solvent-free polymer technologies, such as compounding, conveying and coating in the melt.
Degradation is a collective term for various processes which change the appearance and properties of plastics. Degradation can, for example, be caused by chemical, thermal, oxidative, mechanical or biological influences or also by the effect of rays (such as (uv) light). Examples of consequences are oxidation, chain cleavages, depolymerization, crosslinking or separation of side groups of the polymers. The stability of the polymers with respect to degradation can be increased using additives, for example by adding stabilizers such as antioxidants or photostabilizers.
Uncontrolled degradation often constitutes an unwanted phenomenon. It can be minimized by providing an inert gas atmosphere.
A variety of routes to the solvent-free production a
Hansen Sven
Henke Frank
Hirsch Ralf
Massow Klaus
Neuhaus-Steinmetz Hermann
Norris & McLaughlin & Marcus
Pianalto Bernard
tesa AG
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